Add created vertices to a list
You need to define your vertlist variable outside of the function. Currently, vertlist exists only within the scope of the add_vert function. You can either define it before hand:

vertlist = []
def add_vert(loc):
    ...


or add a try statement to check if vertlist already exists:

def add_vert(loc):
    ...
    try:
        vertlist
    except NameError:
        global vertlist = []
    ...